fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

modules/setting/webhook.go

@@ -34,7 +34,10 @@ func loadWebhookFrom(rootCfg ConfigProvider) {
 	Webhook.QueueLength = sec.Key("QUEUE_LENGTH").MustInt(1000)
 	Webhook.DeliverTimeout = sec.Key("DELIVER_TIMEOUT").MustInt(5)
 	Webhook.SkipTLSVerify = sec.Key("SKIP_TLS_VERIFY").MustBool()
-	Webhook.AllowedHostList = sec.Key("ALLOWED_HOST_LIST").MustString("")
+
+	deprecatedSetting(rootCfg, "webhook", "ALLOWED_HOST_LIST", "security", "ALLOWED_HOST_LIST", "v28.0.0")
+	Webhook.AllowedHostList = sec.Key("ALLOWED_HOST_LIST").MustString(Security.AllowedHostList)
+
 	Webhook.Types = []string{"gitea", "gogs", "slack", "discord", "dingtalk", "telegram", "msteams", "feishu", "matrix", "wechatwork", "packagist"}
 	Webhook.PagingNum = sec.Key("PAGING_NUM").MustInt(10)
 	Webhook.ProxyURL = sec.Key("PROXY_URL").MustString("")